Not Applicable.
Not Applicable.
The present invention relates generally to electrical circuits and, more particularly, to resonant inverter circuits.
There are many types of circuits for powering a load. One such circuit is a resonant inverter circuit, which receives a direct current (DC) signal, from a rectifier for example, and outputs an alternating current (AC) signal. Resonant inverter circuits are used in a wide variety of devices, such as lamp ballasts. The AC output can be coupled to a load, such as a fluorescent lamp, or to a rectifier so as to form a DC-DC converter.
Resonant inverter circuits can have a variety of configurations. For example, a half-bridge inverter circuit includes first and second switching elements, such as transistors, coupled in a half-bridge configuration. A full-bridge inverter circuit includes four switching elements coupled in a full-bridge configuration. Half-bridge and full-bridge inverter circuits are typically driven at a characteristic resonant frequency determined by the impedance values of the various circuit elements, including a resonant inductive element.
Conventional ballast circuits typically include an output transformer inductively coupled to the resonant inductive element for isolating lamps from the resonant circuit. The output transformer is a well known configuration for meeting applicable Underwriters Laboratories (UL) lamp ballast ground fault standards. In general, the current from the ballast lamp terminals is limited to a predetermined level with respect to ground. By limiting the current, a person touching the lamp terminal so as to form a path to ground through the person""s body is not electrocuted.
FIG. 1 shows a typical prior art ballast circuit 10 having a conventional output isolation transformer 12. A rectifier/filter 14 receives an AC input signal on first and second input terminals 16a,b and provides positive and negative voltage rails 18,20. Inductively coupled inductors L1-A, L1-B can be provided on the respective positive and negative rails 18, 20. First and second switching elements 22,24 are coupled across the rails in a well known half-bridge configuration. A primary winding 26, e.g., 1.5 mH 50 turns, of the output isolation transformer combines with a resonating capacitor 28 to form a parallel resonating circuit. A secondary winding 30, e.g., 100 turns, of the transformer energizes first and second lamps LP1, LP2 each of which is coupled in parallel with respective lamp capacitors CL1, CL2. In this well known configuration, the secondary winding 30 of the transformer isolates the lamp terminals from the resonating circuit so as to limit the ground fault current flow. In the event a technician inadvertently touches a lamp terminal and thereby provides a current path to ground, the current flow through the technician""s body is limited to a safe level to prevent injury. Underwriter""s Laboratories promulgates standards for acceptable ballast ground fault current levels.
While the output isolation transformer provides safety, it is relatively bulky so as to require significant space on the ballast circuit board. The output transformer also consumes a relatively high amount of power. In addition, the transformer performance is negatively impacted in some applications by the corona effect. For example, in so-called instant start ballasts, in which a relatively high voltage, e.g., 500 VRMS, is applied to the lamp terminals to initiate current flow through the lamp, the transformer must provide this voltage to strike the lamp. Such a voltage can cause the transformer operating characteristics to degrade over time.
It would, therefore, be desirable to provide a ballast circuit having an enhanced output isolation configuration.
The present invention provides a circuit including a resonant inverter having a relatively efficient and reliable output isolation transformer circuit. In general, the output isolation transformer includes at least one secondary winding that combines with the primary winding to provide the required lamp strike voltage while limiting ground fault current from the lamp terminals. With this arrangement, the required voltages are efficiently applied to the lamps to initiate current flow without compromising safety, e.g., meeting applicable ballast safety standards. While the invention is primarily shown and described in conjunction with ballast circuits, it is understood that the invention is applicable to other circuits, such as power supplies and electrical motors, in which it is desirable to isolate a load and limit ground fault current.
In one aspect of the invention, a resonant circuit includes an output isolation output transformer having a first secondary winding coupled to one of the lamp terminals. A primary winding of the transformer provides a series circuit path with the first secondary windings such that a node at AC ground is disposed between the primary winding and the first secondary winding. The primary winding of the output isolation transformer can also provide an inductor forming a part of the resonating circuit. Further secondary windings can be provided as desired.
In one particular embodiment, a second secondary winding is coupled between the primary winding and the lamp. The voltage across the first secondary winding is applied to one end of the lamp and the voltages across the second secondary winding and the primary winding are applied to the other end of the lamp. The ground fault voltage from a first lamp terminal corresponds to the voltage of the first secondary winding and the ground fault voltage from the second lamp terminal corresponds to the combined voltages of the second secondary winding and the primary winding.
In another aspect of the invention, the circuit includes a feedback path from a point proximate the lamp for reducing harmonic distortion and increasing overall efficiency. In an exemplary embodiment, the circuit includes a feedback path from a closed current loop including a transformer winding to a high frequency rectifier for promoting linear operation of a low frequency input rectifier.